The present invention relates to optical monitoring, and more particularly to a system and method of monitoring the performance of dense wavelength division multiplexing optical communication services.
In densely packed WDM systems (dense WDM, DWDM) messages are communicated by light signals at different wave lengths via a single fiber only. Each wave length is the carrier of an information signal. All channels are within the wave length range from presently roughly 1,520 nm to 1,565 nm. The inter-channel separation amounts to a few nanometers or some hundreds of picometers, respectively. For standardization of these telecommunication systems, the international ITU-T Working Group has recommended the wave lengths (corresponding to the channels) to be used with an inter-channel separation of 100 GHz (□0.8 nm) as standard. The ongoing development of these DWDM systems aims at the extension of the utilizable wave length range up to 1,610 nm for example.
Systems for the continuous monitoring of all characteristic parameters with the possibility of signal regeneration or improvement are required at many sites of this communication system. The most important parameters include the wave length and the capacity of all channels, the monitoring of the line width and the wave length drift of the lasers as well as the signal-to-noise ratio in each communication channel. Typical specification requirements for monitoring are:                wave length measurement per channel with an absolute precision of 0.08 nm and a resolution of 0.01 nm,        power metering per channel with an absolute precision of 0.4 dB and a resolution of 0.1 dB,        S/N measurement between the channels with an absolute precision of 0.4 dB at 0.1 dB,        reproducibility and a dynamic ratio of 33 dB at minimum,        reliability over 1010 measuring cycles (20 years approximately),        low PDL (0.1 dB max.),        small physical size.        
Fundamentally different methods are suitable for monitoring purposes, which are employed in conventional optical spectrum analyzers.
Tunable narrow-band filters are used for wave length selection in the filtering technique. Acousto-optical filters (e.g. those produced by Wandel & Goltermann) or piezo-electrically controlled micro filters (e.g. those from the Queensgate company) or tunable fiber Bragg gratings (e.g. those from ElectroPhotonics Corp.) are applied, which can be tuned directly via an electrical parameter.
The filtering technique is not only restricted to the optical filtering operation but it may also be performed at the electrical signal level after a preceding conversion into electronic signals. With electronic filtering, the optical signal is mixed with an optical reference signal in a non-linear optical component while the differential frequencies are analyzed on an electronic spectral analyzer (Hewlett Packard Co.).
Another variant is the grating monochromator technique wherein either the grating is rotated or the spatially resolved signal spectrum is sensed by means of a single photodiode, or the grating is stationary and a scanning deflection mirror is provided in front of the exit slit of the monochromator, or a mobile reflecting element varies the angle of incidence of the radiation on the grating (e.g. Photonetics company), or a stationary grating is used in combination with a line of photodiodes as detector unit (e.g. Yokogawa company).
In the interferometric technique, the spectrum is obtained from the detector signal of a Michelson interferometer with variable optical paths, with application of the Fourier transform (e.g. Hewlett Packard company).
None of the aforementioned conventional systems is suitable to satisfy the high demands made on a monitoring module for a DWDM system in terms of resolution, measuring accuracy, ASE measurement and dynamic ratio, at the same time and in a suitable manner and to satisfy moreover the demands in terms of short measuring intervals, longevity and low space requirements as well as low-cost realization.
What is desired is a suitable measuring system that satisfies the demands on a DWDM monitoring system in terms of resolution, measuring accuracy, ASE measurement and dynamic ratio, short measuring intervals, longevity and low space requirements as well as a low-cost production.